Quantum Biology of the PSU

It is through photosynthesis that earth's biosphere derives its
energy from sunlight. Photosynthetic organisms, i.e., plants, algae and
photosynthetic bacteria, have developed efficient systems to harvest the
light of the sun and to use the light energy to drive their metabolic
reactions, such as the reduction of carbon dioxide to sugar. The
ubiquitous green color of plants is testimony to the key molecular
participant in the light harvesting of plants, chlorophylls. More hidden
in this respect, but no less widespread, is a second participating
molecule, carotenoid. In green leaves the color of the carotenoids
is masked by the much more abundant chlorophylls while in red ripe
tomatoes or petals of yellow flowers, the carotenoids predominate.
Chlorophyll molecules exist in slightly different chemical structures
in various photosynthetic organisms, as chlorophyll a or b in plants
or algae, and as bacteriochlorophyll a (BChl-a) or b in photosynthetic
bacteria. Molecules such as chlorophyll and carotenoid that absorb
light and impart color to living matter and other materials are called
pigments.

In general, biological pigments are non-covalently bound to proteins,
forming the so-called pigment-protein complexes. The pigment-protein
complexes are organized as the photosynthetic unit (PSU). The
bacterial PSU consists of two types of pigment-protein complexes:
the photosynthetic reaction centers (RCs) and the light-harvesting
complexes. The main function of the light-harvesting complexes is to
gather light energy and to transfer this energy to the reaction centers
for the photo-induced redox processes. In most purple bacteria, the
photosynthetic membranes contain two types of light-harvesting complexes:
light harvesting complex I (LH-I) and light harvesting complex II
(LH-II). While LH-I is tightly bound to the photosynthetic reaction
centers, LH-II is not directly associated with the reaction centers, but
transfers energy to the reaction centers via LH-I.

Purple bacteria are great masters of harvesting light. Nearly all
the energy gained by the absorption of a photon is transferred on to the
reaction center. To illustrate how purple bacteria achieve such high
efficiency, we trace the way of a photon (and its excitation energy,
respectively) through the light-harvesting system. On this way we will
point out the remarkable geometrical features that serve the process
of harvesting light. It is the goal of our research to understand how
these geometrical features translate into physical properties that
ideally support the biological function. It will be shown that purple
bacteria exploit elegant quantum physics, the working of which were
only fully understood recently after the discovery of the structures
of light-harvesting complexes and investigations into their electronic
excitations.

Primary Absorption of a Photon

Light is absorbed either by bacteriochlorophylls or carotenoids in
different spectral regions. Two kinds of bacteriochlorophylls absorb at
slightly different energies and at different angles. The ring structure
enhances absorption and generates an energy trap.

Carotenoid-Chlorophyll Excitation Transfer

In addition to photoprotecting chlorophylls, carotenoids also
enhance the absorption crossection by absorbing light in the green light
range. They transfer their electronic excitation within about 100 fs to
the B850 BChl ring.

B800-B850 BChl Excitation Transfer

B800 BChls transfer excitation via the Foerster mechanism to the B850
BChl ring. The transfer occurs within less than one picosecond. The ring
structure accelerates this transfer by enhancing the spectral overlap
through exciton splitting.